KEYS/DNS: Fix ____call_usermodehelper() to not lose the session keyring

____call_usermodehelper() now erases any credentials set by the
subprocess_inf::init() function.  The problem is that commit
17f60a7da150 ("capabilites: allow the application of capability limits
to usermode helpers") creates and commits new credentials with
prepare_kernel_cred() after the call to the init() function.  This wipes
all keyrings after umh_keys_init() is called.

The best way to deal with this is to put the init() call just prior to
the commit_creds() call, and pass the cred pointer to init().  That
means that umh_keys_init() and suchlike can modify the credentials
_before_ they are published and potentially in use by the rest of the
system.

This prevents request_key() from working as it is prevented from passing
the session keyring it set up with the authorisation token to
/sbin/request-key, and so the latter can't assume the authority to
instantiate the key.  This causes the in-kernel DNS resolver to fail
with ENOKEY unconditionally.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Eric Paris <eparis@redhat.com>
Tested-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 15 insertions, 14 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 97e0d52d72fd..6075a1e727ae 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1996,7 +1996,7 @@ static void wait_for_dump_helpers(struct file *file)
  * is a special value that we use to trap recursive
  * core dumps
  */
-static int umh_pipe_setup(struct subprocess_info *info)
+static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
 {
         struct file *rp, *wp;
         struct fdtable *fdt;
diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index d4a5c84c503d..0da38cf7db7b 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -45,7 +45,7 @@ static inline int request_module_nowait(const char *name, ...) { return -ENOSYS;
 #endif
 
 
-struct key;
+struct cred;
 struct file;
 
 enum umh_wait {
@@ -62,7 +62,7 @@ struct subprocess_info {
         char **envp;
         enum umh_wait wait;
         int retval;
-        int (*init)(struct subprocess_info *info);
+        int (*init)(struct subprocess_info *info, struct cred *new);
         void (*cleanup)(struct subprocess_info *info);
         void *data;
 };
@@ -73,7 +73,7 @@ struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
 
 /* Set various pieces of state into the subprocess_info structure */
 void call_usermodehelper_setfns(struct subprocess_info *info,
-                    int (*init)(struct subprocess_info *info),
+                    int (*init)(struct subprocess_info *info, struct cred *new),
                     void (*cleanup)(struct subprocess_info *info),
                     void *data);
 
@@ -87,7 +87,7 @@ void call_usermodehelper_freeinfo(struct subprocess_info *info);
 static inline int
 call_usermodehelper_fns(char *path, char **argv, char **envp,
                         enum umh_wait wait,
-                        int (*init)(struct subprocess_info *info),
+                        int (*init)(struct subprocess_info *info, struct cred *new),
                         void (*cleanup)(struct subprocess_info *), void *data)
 {
         struct subprocess_info *info;
diff --git a/kernel/kmod.c b/kernel/kmod.c
index ad6a81c58b44..47613dfb7b28 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -156,12 +156,6 @@ static int ____call_usermodehelper(void *data)
          */
         set_user_nice(current, 0);
 
-        if (sub_info->init) {
-                retval = sub_info->init(sub_info);
-                if (retval)
-                        goto fail;
-        }
-
         retval = -ENOMEM;
         new = prepare_kernel_cred(current);
         if (!new)
@@ -173,6 +167,14 @@ static int ____call_usermodehelper(void *data)
                                              new->cap_inheritable);
         spin_unlock(&umh_sysctl_lock);
 
+        if (sub_info->init) {
+                retval = sub_info->init(sub_info, new);
+                if (retval) {
+                        abort_creds(new);
+                        goto fail;
+                }
+        }
+
         commit_creds(new);
 
         retval = kernel_execve(sub_info->path,
@@ -388,7 +390,7 @@ EXPORT_SYMBOL(call_usermodehelper_setup);
  * context in which call_usermodehelper_exec is called.
  */
 void call_usermodehelper_setfns(struct subprocess_info *info,
-                    int (*init)(struct subprocess_info *info),
+                    int (*init)(struct subprocess_info *info, struct cred *new),
                     void (*cleanup)(struct subprocess_info *info),
                     void *data)
 {
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index d31862e0aa1c..8e319a416eec 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -71,9 +71,8 @@ EXPORT_SYMBOL(complete_request_key);
  * This is called in context of freshly forked kthread before kernel_execve(),
  * so we can simply install the desired session_keyring at this point.
  */
-static int umh_keys_init(struct subprocess_info *info)
+static int umh_keys_init(struct subprocess_info *info, struct cred *cred)
 {
-        struct cred *cred = (struct cred*)current_cred();
         struct key *keyring = info->data;
 
         return install_session_keyring_to_cred(cred, keyring);